Generally, hydrodynamic fluid film bearings operate on the principle that a rotating member, such as a shaft or thrust runner, and an adjacent element, such as a smooth foil or the like, establish and maintain a pressurized fluid film layer therebetween. The fluid film layer, also referred to in the industry as a fluid film wedge, provides lubricant-free support for the rotating member. Often, a resilient backing means or spring is disposed between a foil and a stationary member (e.g., a cartridge, retainer or base) in which the rotating member is axially disposed to accommodate deflections of the foil due to the pressurization thereof by the fluid film layer and excursions of the rotating member due to bearing loading and imbalances thereof such as whirl and the like.
Successful bearing designs have used multiple foil layers in the form of a single coiled foil or multiple foil elements. A multi-layer foil construction is advantageous in certain respects. For example, a multi-layer foil is believed to enhance coulomb damping which provides energy dissipation particularly suitable for hydrodynamic bearings. Moreover, such multi-layer foil constructions exhibit larger rotor excursion tolerance, good accommodation of differential expansion of the foils, accommodation of manufacturing misalignments, accommodation of foil imperfections, tolerance to contaminants, and good wipe-wear characteristics at high speeds, as well as during starting and stopping.
A generally accepted foil design in the industry, generally disclosed and claimed in U.S. Pat. Nos. 4,415,280 and 4,415,281, incorporated herein by reference, is shown in FIG. 1. For example, a hydrodynamic fluid film journal bearing 10 includes a stationary retaining member or cartridge 12, also known as a shell, retainer or base, which encloses a rotary shaft or journal 14 adapted for connection to a high speed rotor. The retaining member 12 and the journal 14 combine to define an annular spacing or clearance 16. The retaining member includes an inner surface having a longitudinally extending keyway 18 formed therein.
A first open or split, generally cylindrical-shaped, smooth foil element 20 is disposed within the annular spacing 16 and fixed along an edge to a side of a key 22 slidably received within the keyway 18. A second open or split, generally cylindrical-shaped, smooth foil element 24 is provided inwardly of and concentric to the first foil element 20 within the annular spacing 16. The second foil element 24 is also fixed along an edge to the key 22. In accordance with the arrangement shown in FIG. 1, the first foil element 20 may be referred to as an intermediate or contact foil, while the second foil element 24 may be referred to as a top foil. During starting and stopping of rotation of the journal 14, the top foil often rubs against the journal 14 until sufficient fluid film is created. Accordingly, the top foil is commonly provided with a low-friction coating on the radial inner surface.
A corrugated resilient backing member or spring 26 is disposed within the annular spacing 16 between the retaining member 12 and the foil elements 20, 24. The spring 26 is also of a generally open or split cylindrical shape and fixed along an edge to the key 22. In general, the foil elements 20, 24 are attached to the key 22, and are mounted within the retaining member 12, such that the extension of the first foil element 20 is in a direction opposite that of the second foil element 24.
Since the foil elements 20 and 24 and the spring 26 are discrete members and are not coiled from a single foil element, telescoping of these members due to axial loading and resulting in possible damage to the foils by rubbing with the journal 14 is minimized. Additionally, the provision of discrete foil elements, rather than a single coiled foil allows foils of varying properties of thickness, resilience and the like to be used for precise tailoring of properties.
Hydrodynamic fluid film bearings, such as those of the design shown in FIG. 1, are commonly used in turbo-compressor units employed in modern air cycle machinery for aircraft cooling and ventilation. While multi-layer foil designs exhibit the above-identified advantages, improvements in such bearings in the areas of manufacturability, damping characteristics and compliancy, ease in assembly/disassembly and foil replacement, resistance to foil telescoping, and convenience in providing multi-thickness foil layers are continually being sought. Moreover, the key arrangement shown in FIG. 1 has several drawbacks. Most notably, satisfactory operation of the bearing requires perfection in manufacture of the foils. That is, the key must be perfectly straight and the foils must be attached perfectly square to the key. Any gaps, curves or misalignment will affect the load carrying capacity of the bearing, and may also lead to premature failure of the bearing. Because there are at least two foil elements attached to the key, alignment is more critical. Any imperfections in the key, the foil elements, or the welds will affect operation of the bearing. It is often cumbersome and difficult, and hence expensive, to attach thin foil elements to the small cross-sectional area of the key.
For satisfactory operation of the bearing, the key must also be maintained perfectly straight within the keyway of the retaining member. Keys are susceptible to bending during both manufacture and operation. Further, the key may tilt or twist within the keyway during operation in response to forces exerted on the foil elements. Any bends or twists in the key lead to reductions in load carrying capacity and compliancy of the bearing, and may ultimately cause of failure.
Hydrodynamic fluid film bearings work because of their compliancy. In the key design of FIG. 1, it has been determined that the bearing has low compliancy in the direction of the key. In effect, there is a “hard spot” over the key. With respect to compliance, this hard spot conflicts with the nature of the bearing and the fluid film created therein during rotation of the journal. As a result, the bearing has less load capacity in the direction of any hard spots, namely the location of the key. Further, the bearing has less shock load resistance and damping in that direction.
The present invention, as detailed in the illustrations and description below aims at reducing or eliminating these drawbacks in hydrodynamic fluid film bearings.